I own a time share from Vidanta, specifically the Grand Mayan. They have hotels in about 6 different tourist stops in Mexico, including Los Cabos. I have stayed at the Riviera Maya [20 minutes from the Cancun hotel strip] location about 4 times because I like it so much. They have lots of on premise restaurants, a golf course, nightly themed entertainment, pools all over the place, very kid friendly, the staff treats guests extremely courteous.
Again, my opinion is based on my experience with the Riviera Maya location, but I can tell you that as long as you don't stay in their cheap tier hotels [they have tiers that go from budget to extreme luxury] you won't be disappointed. Here is the link:
http://www.thegrandmayan.com/los-cabos
You can also go to
http://www.grupovidanta.com/organization and see all of the hotels they have. Click on a name to view where in mexico they have locations. I would recommend not looking at the hotels below The Bliss.
When you mentioned Melia, did you mean the hotel chain? If so, they have hotels in the Dominican Republic and the one I stayed in was awesome.
Riu Hotels, Hmmm... I haven't hear anything good from my friends that stayed in the one located in Cancun; the ones in the Dominican Republic are cheap tier hotels, so judging from that I would say skip them.